You need to possess the necessary skills required for the shift.
In order to replace another freelancer, you deem to possess the necessary skills required to apply for the shift. For example: the shift falls under “catering” and you need to have the skills of “walking out three plates” and “Dutch”, these skills have to be verified in your profile.
When you miss one of these required skills, the shift will not be visible in the job overview for you. This means that when you click a replacement link, the link will not show you the shift.
In the case where the shift is not visible for you, this means that you don’t meet the requirements for this specific shift and thus will not be able to apply as a replacement.